I'm incorrect above, the cigarette lighter is on KP-I, not KP-II, so not ideal as the relay would always be drawing a little power in accessory mode when the engine isn't running. A good nearby KP-II fused source would then be tapping into the window/mirror defrost switch, the blue/yellow wire (both 700s and 900s). Ground can be grabbed off the black wire there.

To repeat myself from elsehere in this thread so as to make this branch of the discussion complete for others, I just went out and checked my 940s. Neither wire harness has the unused coolant level connector I mentioned near the reservoir like the 960s. The 700s are likely similar without the connector. However, my 904s do have that unused washer level 2-pin connector down by the bottle with the blue/white wire that I mentioned (other wire is black to ground). That wire goes up to the unused instrument cluster dash warning light. You could sever that wire at the cluster connector and splice in a wire to your relay, saving you having to route any wiring through the firewall. If you can set up a float switch to work normally and not needing a relay, it would make it the same as a 960. You can either sever the blue/white wire there or even try pushing out the pin and moving it over to that unused coolant level warning light I mentioned. All you would then need there would be a square twist lock dash bulb.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now